In recent years, optical communication network enabling large capacity data communication at high speed is expanding. The optical communication network is assumed to be mounted on a commercial-off-the-shelf device in the future. An electrical input/output optical data transmission cable (optical cable) capable of being used no different from the present electrical cable is desired for the application of large capacity data transfer at higher speed, noise countermeasures, and data transmission between substrates in the device. In view of flexibility, a film light guide is desirably used for the optical cable.
The light guide is formed by a core having a large index of refraction and a clad having a small index of refraction arranged contacting the periphery of the core, and propagates the optical signal entered to the core while repeating total-reflection at the boundary of the core and the clad. The film light guide has flexibility since the core and the clad are made of a flexible polymer material.
The light transmission module using the film light guide having flexibility for the optical cable generally includes the following members. In other words, the light transmission module includes a photoelectric conversion element (light emitting and receiving element) that optically couples with the light guide, a substrate including an electrical wiring to be connected to the photoelectric conversion element, and electrical connecting means for electrically connecting the electrical wiring and the photoelectric conversion element. The light emitting and receiving element is an element having a function of converting the electric signal to the optical signal and emitting the same, and receiving the optical signal and converting to the electric signal. In the conventional light transmission module, a wire bonding is used for the electrical connecting means for electrically connecting the electrical wiring and the photoelectric conversion element (e.g., patent document 1).
FIG. 18(a) is a perspective view of the conventional light guide module 201, FIG. 18(b) is a side cross-sectional view thereof, and FIG. 18(c) is a perspective view of an air tightly packaged light guide module 201.
As shown in such figures, a light guide film 210 is mounted on a sub-mount 220, and transmission of light is carried out with a light emitting and receiving element 230 through mirror surface reflection at the end face. The sub-mount 220 for mounting the light guide film 210 is formed in an IC package 240. The IC package 240 is formed with an electrode 242, which electrode 242 is electrically connected to a light emitting and receiving point 232 of the light emitting and receiving element 230 by a wiring 246. In other words, the electrode 242 and the light emitting and receiving point 232 are electrically connected through wire bonding by the wiring 246.
Another example of a configuration of the light transmission module includes a light transmission module disclosed in patent documents 2 and 3.
As shown in FIG. 19, the light transmission module disclosed in patent document 2 has a configuration in which a film optical wiring 311 and an optical device 314 are adhered by a bump 310.
Patent document 3 discloses a technique for determining the positions of the optical element and the light guide by a positioning pin formed in the substrate.
For instance, miniaturization and lower height of the light transmission module are assumed to be required when using the light guide in the small and thin device. However, in the conventional light transmission module shown in FIG. 18, a space for wire bonding needs to be ensured, and thus there is a limit to lowering height.
In other words, as shown in FIGS. 18(a) to 18(c), the wiring 246 has a loop structure since a high and low difference is created between the forming surface of the light emitting and receiving point 232 and the forming surface of the electrode 242 when electrically connecting the wiring 246 and the light emitting and receiving point 232 by wire bonding. Thus, in the conventional light transmission module, a design that ensures the loop height of the wiring 246 (distance between the forming surface of the light emitting and receiving point 232 and the vertex of the loop structure of the wiring 246) is made. Therefore, setting the optical coupling distance L (distance between the light emitting and receiving point 232 and the light guide film 210) to smaller than or equal to the loop height of the wiring 246 becomes difficult when mounting the light guide film 210 on the sub-mount 220, and thus there is a limit to lowering the height of the light transmission module.
The light transmission module using the film light guide for the optical cable converts the electric signal to the optical signal and emits the same, and receives the optical signal and converts to the electric signal in the photoelectric conversion element (light emitting and receiving element). Thus, compared to the signal transmission module using a normal electrical wiring, the light transmission module consumes extra power in optical—electric conversion or electric—optical conversion in the photoelectric conversion element, and hence the power consumption increases.
Furthermore, the light quantity of the light exit from the light emitting element towards the film light guide needs to be increased to ensure the light quantity that enters from the film light guide to the light receiving element the greater the optical coupling distance in the light transmission module, and hence the power (power consumption) necessary in the light emitting and receiving element increases. In the conventional light transmission module shown in FIGS. 18(a) to 18(c), setting the optical coupling distance L to smaller than or equal to the loop height of the wiring 246 is difficult, and thus reduction of the power consumption by reduction of the optical coupling distance L is also limited.
If the light guide film 210 is mounted avoiding the loop structure of the wiring 246, a loop arrangement area of the wiring 246 needs to be ensured in the IC package 240, and hence the size of the entire light transmission module becomes large.
An improved configuration for realizing lower height (reduce optical coupling distance L) may be a configuration of electrically connecting the light emitting and receiving element 230 and the electrode 242 with a liquid conductive material such as silver paste and soldering instead of wire bonding by the wiring 246. However, in such improved configuration, the liquid conductive material tends to diffuse and interfere with the light emitting and receiving point 232.    Patent document 1: Japanese Laid-Open Patent Publication “Japanese Laid-Open Patent Publication No. 2005-321560 (date of publication: Nov. 17, 2005).    Patent document 2: Japanese Laid-Open Patent Publication “Japanese Patent Publication No. 3257776 (registered: Dec. 7, 2001).    Patent document 3: Japanese Laid-Open Patent Publication “Japanese Laid-Open Patent Publication No. 2006-259682 (date of publication: Sep. 28, 2006).